Note: Descriptions are shown in the official language in which they were submitted.
CA 0222~337 1997-12-19
Method of Combustion with a Two Stream T~ngential Entry Nozzle
TECHNICAL FELD
This invention relates to low NOx premix fuel nozzles, and particularly to such
nozzles for use in gas turbine engines.
BACKGROUND OF THE INVENTION
The production of nitrous oxides (hereinafter "NOx") occurs as a result of
combustion at high temperatures. NOx and carbon monoxide ("CO") are notorious
pollutants, and as a result, combustion devices which produce NOx and CO are subject to
ever more stringent standards for emissions of such pollutants. Accordingly, much effort
is being put forth to reduce the formation of NOx and CO in combustion devices.
One solution has been to premix the fuel with an excess of air such that the
combustion occurs with local high excess air, resul~ing in a relatively low combustion
temperature and thereby minimi7inf~ the formation of NOx. A fuel nozzle which sooperates is shown in U.S. Pat. No. 5,307,634, which discloses a scroll swirler with a
conical centerbody. This type of fuel nozzle is known as a tangential entry fuel nozzle, and
comprises two offset cylindrical-arc scrolls connected to two endplates. Combustion air
enters the swirler through two subst~nti~lly rect~n~ r slots formed by the offset scrolls,
and exits through a combustor inlet port in one endplate and flows into the combustor. A
linear array of orifices located on the outer scroll opposite the inner trailing edge injects
fuel into the airflow at each inlet slot from a manifold to produce a uniform fuel air
mixture before exiting into the combustor.
Premix fuel nozzles of the tangential entry type operating in at lean fuel/air ratios
have demonstrated low emissions of NOx relative to fuel nozzles ofthe prior art.Unfortunately, fuel nozzles such as the one disclosed in the aforementioned patent have
exhibited combustion instabilities over the normal operating range thereof as a result of
this lean operating condition.
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What is needed is a method of operating a ~n~e~ l entry fuel nozzle in at lean
fuel/air Mtios that achieve the goals of low NOx and low CO emissions without
experiencing the combustion instabilities observed in the prior art.
SIJM~RY OF T~IE INVENTION
It is therefore an object of the present invention to provide a method of ope~ ga tangential entry fuel nozzle in at lean fuel/air ratios that achieve the goals of low NOx
and low CO emissions without experiencing the combustion instabilities observed in the
prior art.
Accordingly, a method for burning fuel in the combustor of a gas turbine engine
with a premixing type of combustion is dicclosed which comprises providing a scroll
swirler having first and second endplates, the first çndpl~e is spaced relation to the second
endplate defining a subst~nti~lly cylindrical mixing zone therebetween, the second çntlpl~t~
having a combustor inlet port extending thelet}lrough, providing a centerbody located
within the mixing zone and having a radially outer surface that tapers toward the
combustor inlet and extends subst~nti~lly the entire length ofthe mixing zone, introdncing
a first portion of combustion air ~ange~,lially into the mixing zone subst~nti~lly
continuously along the length thereof, introducing a first portion of fuel into the
combustion air as the combustion air is introduced into the mixing zone"nixing the
combustion air and fuel by swirling the combustion air and fuel about the centerbody while
flowing the combustion air and fuel towards the combustor inlet, flowing the first portion
of combustion air into the combustor inlet, introducing a second portion of colnh--stion air
into the first portion radially inward thereof at the combustor inlet, the sum of the f~rst and
second portions of combustion air defining total airflow, and the second portion of
combustion air equal to 85-89% ofthe total airflow, and burning the fuel external ofthe
mixing zone.
BRIEF DESCRlPTION THE DRAWINGS
Figure 1 is a cross-sectio~l view of the fuel nozzle of the present invention,
taken along line 1-1 of Figure 2.
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Figure 2 is a cross-sectional view looking down the longitudinal axis of the
nozzle of the present invention.
Figure 3 is a cross-sectional view of the fuel nozzle of the present invention,
taken along line 3-3 of Figure 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1, the low NOx premix fuel nozzle 10 of the present
invention includes a centerbody 12 within a scroll swirler 14. The scroll swirler 14 inCl~ldçs
first and second endplates 16,18, and the first endplate is conne~led to the centerbody 12
and is in spaced relation to the second endplate 18, which has a combustor inlet port 20
extending therelhrough. A plurality, and plefelabl~ two, cylindrical-arc scroll members 22,
24 extend from the first endplate 16 to the second endplate 18.
The scroll members 22, 24 are spaced uniformly about the longitudinal axis 26 ofthe nozzle 10 thereby definin~ a mixing zone 28 therebetween, as shown in Figure 2. Each
scroll member 22, 24 has a radially inner surface which faces the longitudinal axis 26 and
defines a surface of partial revolution about a centerline 32, 34. As used herein, the term
"surface of partial revolution" means a surface generated by rotating a line less than one
complete revolution about one ofthe centerlines 32, 34.
Each scroll member 22 is in spaced relation to the other scroll member 24, and
the centerline 32, 34 of each of the scroll members 22, 24 is located within the mixing
zone 28, as shown in Figure 2. Rerel,i"~ to Figure 3, each ofthe centerlines 32, 34 is
parallel, and in spaced relation, to the loneit~ n~l axis 26, and all of the centerlines 32, 34
are located equidistant from the loneit~din~l axis 26, thereby dçfinine inlet slots 36, 38
extçndin~ parallel to the loneit~lrlin~l axis 26 between each pair of adjacent scroll mçmbers
22, 24 for introducing combustion air 40 into the mixing zone 28. Combustion supporting
air 42 from the co,np~ssor (not shown) passes through the inlet slots 36, 38 formed by
the overlapping ends 44, S0, 48, 46 ofthe scroll members 22, 24 with offset centerlines
32, 34.
Each of the scroll members 22, 24 further inGludçs a fuel conduit 52, S4 for
introducing fuel into the combustion air 40 as it is introduced into the mixing zone 28
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through one ofthe inlet slots 36, 38. A first fuel supply line (not shown), which may
supply either a liquid or gas fuel, but preferably gas, is connected to the each of the fuel
conduits 52~ 54. The combustor inlet port 20, which is coaxial with the longiturlin~l axis
26, is located immediately adjacent the combustor 56 to discharge the fuel and combustion
air from the present invention into the combustor 56, where combustion of the fuel and air
takes place.
Referring back to Figure 1, the centerbody 12 has a base 58 that has at least one,
and preferably a plurality, of air supply ports 60, 62 extending therethrough, and the base
58 is perpendicular to the longitudinal axis 26 extending therethrough. The centerbody 12
also has an internal passageway 64 that is coaxial with the longitudinal axis 26. In the
preferred embodiment of the invention, the internal passageway 64 includes a first
cylindrical passage 66 having a first end 68 and a second end 70, and a second cylindrical
passage 72 of greater diameter than the first cylindrical passage 66 and likewise having a
first end 74 and a second end 76. The second cylindrical passage 72 communicates with
the first cylindrical passage 66 through a tapered passage 78 having a first end 80 that has
a diameter equal to the diameter ofthe first cylindrical passage 66, and a second end 82
that has a diameter equal to the ~ metçr of the second cylindrical passage 72. Each of the
passages 66, 72, 78 is coaxial with the lonEitu~in~l axis 26, and the first end 80 of the
tapered passage 78 is integral with the second end 70 of the first cylindrical passage 66,
while the second end 82 of the tapered passage 78 is integral with the first end 74 of the
second cylindrical passage 72. The first cylindrical passage 66 includes a discharge orifice
68 that is circular and coaxial with the longitudinal axis 26, and is located at the first end
68 ofthe first cylindrical passage 66.
Referring to Figure 3, the radially outer surface 84 of the centerbody 12 is
includes a frustum portion 86, which defines the outer surface of a frustum that is coaxial
with the longitudinal axis 26 and flares toward the base 58, and a cylindrical portion 88
which is integral with the frustum portion 86, defines the surface of a cylinder, and is
coaxial with the axis 26. In the plefel,ed embodiment, the cylindrical portion 88
termill~tçs at the plane within which the discharge orifice 68 is located, the r~i~meter of the
frustum portion 86 at the base 58 is 2.65 times greater than the diameter of the frustum
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portion 86 at the apex thereof, and the height 90 of the frustum portion 86 (the dict~nce
between the plane in which the base 58 meets the frustum portion 86 and the plane in
which the apex ofthe frustum portion 86 is located) is apploxi,.,ately 1.3 times the
di~meter of the frustum portion 86 at the base 58. The cylindrical portion 88, which is
located between the frustum portion 86 and the dischar~e orifice 68. As shown in Figure
3, the internal passageway 64 is located radially inward from the radially outer surface 84
of the centerbody 12, the frustum portion 86 is coaxial with the longit~din~l axis 26, and
the centerbody 12 is connected to the base 58 such that the frustum portion 86 tapers
toward, and terminates at the cylindrical portion 88. As shown in Figure 2, the base ofthe
rrustum portion 86 fits within a circle 92 inscribed in the mixing zone 28 and having its
center 94 on the longitudinal axis 26. As those skilled in the art will readily appreciate, the
mixing zone 28 is not circular in cross section.
Referring to Figure 1, an internal c~ ber 100 is located within the centerbody
12 between the base 58 and the second end 76 ofthe second cylindrical passage 72, which
terminates at the chamber 100. Air 102 is supplied to the chamber 100 through the air
supply ports 60, 62 in the base 58 which comnl~nicate therewith, and the chamber 100, in
turn, supplies air to the internal pa~s~ge~ay 64 through the second end 76 ofthe second
cylindrical p~cs~ge 72. The first endplale 16 has openings 104, 106 therein that are aligne
with the air supply ports 60, 62 ofthe base 58 so as not to interfere with the flow of
combustion air 102 from the co,..~,ressor ofthe gas turbine engine. A swirler 108,
preferably of the radial inflow type known in the art, is coaxial with the longitudinal axis
26 and is located within the chamber 100 imme(li~tely adjacent the second end 76 ofthe
second cylindrical passage 72 such that all air entering the internal passageway 64 from
the chamber 100 must pass through the swirler 108.
A fuel lance 110, which likewise is coaxial with the longitudinal axis 26, extends
through the base 58, the chamber 100, and the swirler 108, and into the second cylindrical
pass~e 72 of the internal pass~geway 64. The larger ~i~meter of the second cylindrical
pacs~e 72 acco",,-,odates the cross-sectional area ofthe fuel-lance 110, so that the flow
area within the second cylindrical passage 72 is essçnti~lly equal to the flow area of the
first cylindrical passage 66. A second fuel supply line (not shown), which may supply
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either a liquid or gas fuel is connected to the fuel lance 110 to supply fuel to an inner
passage 112 within the fuel lance 110. Fuel jets 114 are located in the fuel lance 110 and
provide a pathway for fuel to exit from the fuel lance 110 into the internal passageway 64.
Referring to Figure 3 the combustor inlet port 20 is coaxial with the longitudinal
axis 26 and include5 a convergent surface 116 a divergent surface 117 and a cylindrical
surface 118 that defines the throat plane 120 of the inlet port 20. The convergent surface
116 the divergent surface 117 and the cylindrical surface 118 are coaxial with the
longitudinal axis 26 and the convergent surface 116 is located between the first endplate
16 and the cylindrical surface 118. The convergent surface 116 is subst~nti~lly conical in
shape and tapers toward the cylindrical surface 118 while the divergent surface is
prefeMbly defined by rotating a portion of an ellipse about the longit~ in~l axis 26.
The cylindrical surface 118 extends a finite ~list~nce 121 between the throat plane
120 and the divergent surface. The divergent surface 117 e~çnrls between the cylindrical
surface 118 the combustor surface 122 ofthe combustor port inlet 20 which is
perpendicular to the longitudinal axis 26 and defines the exit plane 124 ofthe fuel nozzle
10 of the present invention. To achieve the desired axial velocity of the fuel/air mixture
through the combustor inlet port 20 the combustion air flowing thelethlough mustencounter the minimum fiow area or throat area at the combustor inlet port 20. To
achieve this result the cylindrical surface 118 is located at a predetel ~,h~ed radius from the
longitudinal axis 26 that is at least 10% less than the radius of the frustum portion 86 at
the base 58.
The convergent surface 116 terminates at the throat plane 120 where the
di~meter of the convergent surface 116 is equal to the (~i~meter of the cylindrical surface
118. As shown in Figure 3 the throat plane 120 is located between the exit plane 124 and
the discharge orifice 68 of the internal passageway 64 and the convergent surface 116 is
located between the cylindrical surface 118 and the first endplate 16. In order to establish
the desired velocity profile of the fueUair mixture within the combustor inlet port 20 the
convergent surface 116 extends a predeterrnined distance 126 along the longitudin~l axis
26 and the cylindrical surface 118 extends a second distance 128 along the longit~ldin~l
axis 26 that is at least 5% of the predetermined distance 126.
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In operation, 11- 15% ofthe total airflow through the fuel nozzle 10 is
introduced through the openings 104, 106 and the air supply ports 60, 62 in the base 58
and into the chamber 100 ofthe centerbody 12. The combustion air exits the cha~-~ber 100
through the radial inflow swirler 108 and' enters the internal passageway 64 with a
substantial tangential velocity, or swirl, relative to the longitudinal axis 26. When this
swirling combustion air passes the fuel lance 110, fuel, preferably in gaseous form, is
sprayed from the fuel lance 110 into the internal passage 64 and mixes with the swirling
combustion air. The mixture of fuel and combustion air then flows from the second
cylindrical passage 72 into the first cylindrical pa~s~ge 66 through the tapered passage 78.
The mixture then proceeds down the length of the first cylindrical passage 66, exiting the
first cylindrical passage 66 just short of, or at, the throat plane 120 of the combustor inlet
port 20, providing a central stream of fuel/air mixture.
Additional combustion air equal to 85-89% of the total airflow through the fuel
nozzle 10 is introduced into the mixing zone 28 through the inlet slots 36, 38. As used
herein, the term total airflow means the sum of the combustion air entering through the
inlet slots 36, 38 and the combustion air entering through the air supply ports 60,62. Fuel,
preferably gaseous fuel, supplied to the fuel conduits 52, 54 is sprayed into the combustion
air passing through the inlet slots 36, 38 and begins mixing therewith. Due to the shape of
the scroll members 22, 24, this mixture establishes an annular stream swirling about the
centerbody 12, and the fueVair mixture continues to mix as it swirls thereabout while
progressing along the longitudinal axis 26 toward the combustor inlet port 20. Fuel air
concentrations have been specified in such a fashion that if the overall desired fuel/air ratio
was 0.5 times that required for stoichiometric combustion, then the central stream would
have a fuel/air ratio of 0.54 times stoichiometric and the rest of the flow would have a
fuel/air ratio 0.493 times stoichiometric.
The swirl ofthe annular stream produced by the scroll swirler 14 is preferably co-
rotational with the swirl of the fuel/air mixture in the first cylindrical passage 66, and
preferably has an angular velocity at least as great as the angular velocity of the of the
fuel/air mixture in the first cylindrical pacs~e 66. Due to the shape of the centerbody 12,
the axial velocity of the annular stream is m~int~ined at speeds which prevent the
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combustor flame from migrating into the scroll swirler 14 and attaching to the outer
surface 84 ofthe centerbody 12. Upon exiting the first cylindrical passage 66, the swirling
fuel/air mixture of the central stream is surrounded by the annular stream of the scroll
swirler 14, and the two streams flow radially inward ofthe cylindrical surface 118 and
then the divergent surface 117 until reaching the exit plane 124 ofthe combustion intet
port 20 downstream of the mixing zone 28.
Testing of the fuel nozzle 10 of the present has de",on~L, ~ted lean fueVair ratios
that achieve the goals of low NOx and low CO emissions without experiencing the
combustion instabilities observed in the prior art. Key to the operation of the nozle is the
division of the air and fuel between the two streams. Enough fuel must pass through the
central stream that the overall flame is stabilized by its presence, yet the fuel/air ratio
should not be so high as to cause significant NOx production nor rob the rest of the flame
of fuel. Further, the fuel supplied to the two air streams must be manifolded
and controlled independently, to allow the proportion of fuel in the centrat stream to be
varied during operation in order to obtain optimum emissions.
This invention difEèrs from other piloting and stabilizing methodologies in several
ways. First, this invention is being applied to lean, premixed systems. Both ~I-ea.ns are
premixed, with one stream being only slightly more fuel rich than the other. This produces
significantly lower emissions than the traditionat methodology of piloting ~vith a diffusion
flame. Indeed, the present invention is not "piloting" since its function is not to provide a
flame source in the absence of flame elsewhere but rather to provide a flame with extended
stability characteristics and low emissions.
Second, the two (or more) streams form a single, integrated, unified flame front.
While it may be argued that contiguous flames always form a single flame front, the
eCcence of this invention is the subtle manipulation and control of the fuel species in single
flame structure. In the tested embodiments that were most s~ccescfi~l, the two streams
nearly matched each other in fuel/air ratio, in axial velocity, in rotation, and in
temperature, with the differences being slight (i.e. 10% difference in fuet/air ratio). Thus,
the benefits of fuel lean flames are obtained white lesse~ g some of their restrictions.
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Third, the streams are physically separate and can be controlled indepen-lPntly.Liquid-fuel injéctors often use a differentiation in droplet size or velocity to produce richer
and leaner portions of the flame in order to extend flame stability and reduce emissions.
Similarly, the fuel ports in a lean, premixed, gaseous fuel injector may be differentially
sized or located in order to produce fuel-rich and fuel-lean portions of the flame. Or the
aerodynamics may be so controlled as to produce separ~tion in such a fashion as to
promote a fuel-rich or fuel-lean environment. The invention presented here differs from
these in that the streams are kept physically separate until they nearly enter the combustion
zone, with only enough mixing time permitted to allow the formation of the single,
integrated, unified flame front described above.
Although this invention has been shown and des.,-il.ed with respect to a detailed
embodiment thereof, it will be understood by those skilled in the art that various ch~es in
form and detail thereofmay be made without dep~uling from the spirit and scope ofthe claimed
invention.
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